Of the ten, four are Flamengo youth team players, two are young boys on trial, and four are club employees.
The cause was not immediately known, but local newspaper Extra said witnesses alleged the fire started in the air conditioning system.
Apart from the 10 people reported to have lost their lives in the fire disaster, there are many persons who sustained serious injuries and are being treated in the hospital.
The state of Rio de Janeiro has already decreed three days of mourning following the tragedy as supporters have already begun to leave tributes at the Ninho do Urubu. The fire was reportedly controlled at around 7:20 a.m.
"We are distraught", Flamengo President Rodolfo Landim said outside the complex, where friends, fans and neighbours gathered, some in prayer. "I just spoke to Caix Suarez and he is alive", said Rodrigues, adding that the youth told him he ran when he saw the flames in the morning.
Several of the club's best-known young talents trained there, including midfielder Lucas Paqueta, who made his debut for AC Milan in January, and Vinicius Junior, who joined Real Madrid previous year. The development leagues identify promising players at a young age, working with them as they grow through their teenage years.
The fire broke out at 5 am in the Vargem Grande district of Rio in a modern facility where the top-flight professional Flamengo squad also trains, the news website G1 said.
"It's a sad day for Brazilian soccer", tweeted Pele, the country's most famous player.
This is the second tragedy to hit Rio de Janeiro in the past 48 hours after at least six people died following a storm that caused floods and mudslides on Wednesday night. Let us pray for all! "May the Red-and-Black nation have strength and faith to get through this moment".
"For poor kids, many see it as the only path", he said. In recent years, major clubs such as Vasco da Gama, Botafogo and Parana have hit the headlines due to the precarious conditions of their training centers.
The club spent roughly $6.2 million to expand the training ground known as "Ninho do Urubu" over the past year. In addition to accommodation for young players, the facilities include several pitches, an aquatic park, a gym, a medical centre and a mini stadium.
Photos taken by The Associated Press showed a desperate woman running down a narrow street and the photographer said she was yelling that the police were getting ready to "execute" people inside a nearby house.